It is known that polycrystalline diamond can be deposited as a film by low pressure CVD (chemical vapor deposition) from an activated gas mixture which contains active radical species of the appropriate gaseous constituents needed. These species include a facilitating gas, generally atomic hydrogen, and a carbon source gas, generally a hydrocarbon compound. Such an activated gas is also referred to by some as a "plasma," because it is at a high enough temperature for the gases to be at least partially ionized, although others may consider the level ionization to be too low for it to be considered a true plasma.
There are several means known in the art for providing the energy necessary to form and maintain the activated gas. One is by a heated electric filament. Another is by a microwave generator coupled to an appropriate resonant cavity. A third is by combustion, as by a torch. A fourth is by a direct current electric arc system commonly referred to as an "arc jet." An arc jet deposition system of this type is described, for example, in U.S. Pat. No. 4, 682,564 issued Jul. 28, 1987 to Gordon L. Cann. A major distinction between microwave systems and arc jet systems is that microwave systems have a relatively slow growth rate by comparison. Arc jet systems can have a growth rate of more than ten times that of microwave systems.
It has been observed both in microwave and other deposition systems for diamond film that a reduction in the concentration of the carbon source gas for the activated gas mix dramatically reduces the growth rate, while at the same time, however, also improving the quality of the diamond film by reducing the concentration of imperfections present to thereby increase both the optical transparency and the thermal conductivity. The greatest amount of investigative activity in this regard has been with microwave systems. With these systems it has been observed that with methane concentrations less than about 0.1% (Gas concentration percentages are by volume throughout.), the growth rate is so low as to be of little commercial interest. Consequently, there has been little study of diamond film deposition with methane concentrations lower than this 0.1% level.